Studies are being made on polyimide foams because they are expected to have excellent properties, such as heat resistance, compared to other polymer foams. It is, however, difficult to produce polyimide foams, especially foams made of aromatic polyimides, and only limited types of aromatic polyimides with specific chemical structures can produce foams for practical use.
Patent Documents 1 and 2 disclose methods for producing polyimide foams using 3,3′,4,4′-benzophenone tetracarboxylic acid diesters as tetracarboxylic acid components. The polyimide foams disclosed as preferred or concrete examples therein employ a combination of an aromatic diamine and a heterocycle diamine as the diamine components and have glass transition temperatures of 300° C. or below.
Patent Document 3 discloses the production of a polyimide foam wherein: a 3,3′,4,4′-biphenyltetracarboxylic acid diester is complexed with an ether such as THF (by hydrogen bonding) to form a uniform solution with an aromatic diamine; and this uniform solution is used to prepare a polyimide precursor, from which the polyimide foam is produced. The disclosed polyimide foam made from the 3,3′,4,4′-biphenyltetracarboxylic acid diester, however, has extremely coarse and non-uniform foam cells. The disclosed polyimide foam also lacks the mechanical properties required for practical use as a foam—e.g., it cannot deform easily and has poor cushioning properties because its expansion ratio is small (i.e., its apparent density is large). Further, either 3,4′-oxydianiline alone, or a combination of 3,4′-oxydianiline and 1,3-bis(3-aminophenoxy)benzene, is used as the diamine component(s), which respectively give glass transition temperatures of 250° C. and 261° C.
Patent Document 4 discloses a method for producing a polyimide foam employing a 2,3,3′,4′-biphenyltetracarboxylic acid diester as a tetracarboxylic acid component. The diamine components concretely disclosed therein are a combination of an aromatic diamine, such as p-phenylenediamine, and diaminosiloxane.    Patent Document 1: JP-A-59-145222    Patent Document 2: JP-A-4-211440    Patent Document 3: JP-T-2000-515584    Patent Document 4: JP-A-2002-12688